docs: def __exec_python_cmd(cmd):
      """
      Executes an externally spawned Python interpreter and returns
      anything that was emitted in the standard output as a single
      string.
      """
      # Prepend PYTHONPATH with pathex
      pp = os.pathsep.join(PyInstaller.__pathex__)
      old_pp = compat.getenv('PYTHONPATH')
      if old_pp:
          pp = os.pathsep.join([pp, old_pp])
      compat.setenv("PYTHONPATH", pp)
      try:
          try:
              txt = compat.exec_python(*cmd)
          except OSError, e:
              raise SystemExit("Execution failed: %s" % e)
      finally:
          if old_pp is not None:
              compat.setenv("PYTHONPATH", old_pp)
          else:
              compat.unsetenv("PYTHONPATH")
      return txt.strip()